1. Field of the Invention
The present invention relates generally to a fuel injection system for an internal combustion engine and more specifically to such a system which enables accurate real time control of the amount of fueld which is to be injected per cylinder by approximating, at/or prior to the beginning of each induction phase, the the total amount of air which will be charged into each cylinder of the engine during the instant induction phase.
2. Description of the Prior Art
A previously proposed injection control system for an internal combustion engine has been disclosed in an article entitled `Development of the Toyota Lean Combustion System` published in `NAINEN KIKAN` Vol. 23 Oct. 1984 issue pages 33 to 40. This system strives to control the air-fuel ratio of the air-fuel mixture charged into the cylinders of the engine over a wide range spanning approximately stoichiometric to lean mixtures. In order to initially determine the appropriate air-fuel mixture, the output of an induction pressure sensor is used to sense how much air is being inducted into the engine. Subsequently, to enable feed-back control of the injection volume a specially developed air-fuel ratio sensor capable of sensing air-fuel ratios until the mixtures become super lean is used.
In this system, because the amount of fuel supplied to the engine varies with the load thereon it is necessary to correct the output of the pressure sensor before using the same in the appropriate calculations.
However, even though the pressure sensor output matches the actual induction air flow reasonably accurately, the derivation of the injection amount per cylinder, although not critical under most modes of operation, becomes inadequate when leaner mixtures are involved, namely, mixtures leaner than those (eg. super lean mixtures) which can be accurately sensed by the air-fuel ratio sensor and corrected by feed-back control.
The calculation of the amount of fuel required is carried out in a microprocessor at a predetermined timing prior to actual injection. In order to provide sufficient time for the calculation, the output of the pressure sensor is read at a time prior the start of the induction phase (e.g. at a time t.sub.1 - see FIG. 5).
However, as will be appreciated from FIG. 5, the amount of air continues to be introduced into the cylinder at least until time t.sub.3 (the end of the induction phase) depending on the valve overlap and ramming characteristics of the induction system, while the injection of fuel terminates at a time t.sub.2. As will be appreciated that the actual amount of air inducted into the cylinder and which mixes with the fuel therein is more accurately represented by the pressure sensor output which occurs at time t.sub.3 (noting that PB1&lt;PB3).
This of course means that the correction according to the reading of the pressure sensor at time t.sub.1 is not really effective and thus leaves the system completely dependent on the air-fuel ratio sensor feed-back control and renders the same unable to improve the control level sufficiently rapidly to that which will be necessary in the near future in order to meet stricter emission control standards which will become mandatory at that time.